Sitting for long periods of time can cause all kinds of physical problems. In my article Piecing All Day Was Bad on My Back, I talked about the Pomodoro Method of taking short breaks to be more productive. Even if you're taking multiple short breaks, it's still good to have a comfortable place to sit. I often use the Sit Upon sewing chair pad by the Gypsy Quilter to make my chair more comfortable. It is soft and adds a few inches of height as you sit.
This is especially useful when going to classes where the only chairs are of the metal, folding variety. There is no need for me to drag my sewing chair to retreats and classes if I bring the Sit Upon. Watch the video to see my review.